Laser and fiber coupling control

ABSTRACT

One or more single mode waveguide devices are fiber coupled such that signals to an optical element affect the coupling of the waveguide device to an optical fiber. A number of systems and methods are disclosed to adjust the coupling of the waveguide device to the optical fiber. These include dithering the tunable optical element at different frequencies along differing axes and using a lock-in technique to derive an error signal for each degree of motion, using a beamsplitter to form a secondary image of the focused beam on a position-sensitive detector, the use of a chiseled fiber to generate reflections from the angled facets, using an additional laser for a secondary image, or obtaining a secondary image from an angled fiber or a parasitic reflection.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application,Ser. No. 10/002,703, filed Oct. 30, 2001 now U.S. Pat. No. 6,771,855 andclaims the benefit of U.S. provisional application No. 60/244,689 filedOct. 30, 2000, 60/244,738 filed Oct. 31, 2000, 60/311,621 filed Aug. 8,2001, 60/311,443 filed Aug. 8, 2001 and U.S. provisional patentapplication entitled Error Signal For Fiber Coupling, application No.60/340,975, filed Oct. 29, 2001, which are hereby incorporated byreference as if set forth in full herein.

BACKGROUND

The present invention relates generally to lasers and in particular tocontrolling fiber coupling between an array of lasers and an opticaloutput.

Fiber coupling is often an essential but costly step in packagingvarious waveguide devices for telecommunication applications. On accountof the very small optical modes in single mode waveguide devices, verytight submicron tolerances are often required in the packaging.

Generally, the devices are actively aligned. For example to fiber couplea telecommunication laser, the device is activated, and the opticalpower coupled to the fiber is monitored as the positions of the variousoptical elements in the package are varied. When the coupling ismaximized, the optical elements are permanently fixed in position. Theprocess is time consuming, costly, and often not very reproducible dueto contraction in epoxies or thermal expansion of the components.

Furthermore, all the components in the package should be made absolutelyimmobile for the above procedure to maintain effectiveness over time.Any change in the position of the elements decreases the opticalcoupling. This makes hybrid integration of components with varyingexpansion coefficients very difficult. For example, to package a laserwith a lithium niobate modulator, the laser uses hard solder for thermalheatsinking, while the modulator uses a soft epoxy that does not stressthe crystal. The relative position of these devices will vary in thepackage due to the mismatch in the materials. Similarly, solders andepoxies tend to cause stress in the fiber, which affects yield andreliability and can cause birefringence in the fiber that influences thepolarization of light in the core.

BRIEF SUMMARY OF THE INVENTION

The present invention provides adjustable optical coupling systems andmethods. In one embodiment, a laser from an array of lasers is selectedin which each laser emits light at different wavelengths. An opticalpath from the laser to an optical output is established such that lightfrom the laser is transmitted into an optical output. The optical pathestablished is adjusted to maximize output power of the emitted lightinto the optical output. In one aspect of the invention, a look-up tableis established where the table has entries in which individual lasers inthe laser array are each assigned an output power value and an entry inthe look-up table that corresponds to the selected laser is identified.In another aspect of the invention, a look-up table is established wherethe table has entries in which individual lasers in the laser array areeach assigned a predetermined output power value and associated with apredetermined location identified for the optical element. An entry inthe look-up table that corresponds to the selected laser is identified.

In one embodiment, the system comprises an array of lasers, at least oneoptical element and an optical output such that light from a laser fromthe array of lasers is directed into the optical output by the at leastone optical element. A controller is also coupled to the at least oneoptical element and configured to adjust the optical element to maximizeoutput power of the light directed into the optical output. In oneaspect of the invention, the system also comprises a plurality ofphotodetectors proximate the optical output. The controller is coupledto the plurality of photodetectors and is configured to adjust theoptical element based on the information provided by the photodetectors.The information provided by the photodetector comprises optical outputpower of light received at one or more of the photodetector and/or alocation of light incident upon one or more of the photodetectors. Inanother aspect of the invention, the controller generates an errorsignal to adjust the optical element.

In a further embodiment, the system comprises an array of lasers havinglasers configured to emit light, an optical output configured to receivelight and a detector near the optical output. The system also includesat least one optical element configured to receive light from a laserfrom the array of lasers and to direct a portion of the light to theoptical output and a portion of light to the detector. A controller iscoupled to the at least one optical element and configured to adjust theat least one optical element to maximize output power of the lightdirected into the optical output. In one aspect of the invention, theoptical element comprises a beam splitter and/or a mirror.

In a further embodiment of the invention, the system comprises an arrayof lasers comprising a first laser and a second laser where the firstlaser is configured to emit light and the second laser is configured toemit light. An optical output is also provided and configured to receivelight from the first laser. The detector near the optical output isconfigured to receive light from the second laser. Also, at least oneoptical element is provided and configured to receive light from thefirst and second lasers and a controller is coupled to the at least oneoptical element and configured to adjust the at least one opticalelement to maximize output power of the light into the optical output.In other aspects of the invention, the second laser is a predetermineddistance from the first laser.

In another embodiment of the present invention, the system comprisesemitting means for emitting light having differing wavelengths, outputmeans, and optical means for directing light having a particularwavelength from the emitting means into the output means. Coupled to theoptical means is control means that also adjusts the optical means tomaximize power of the light directed into the output means. In anotheraspect of the invention, the system further comprises reflective meansfor reflecting light from the emitting means and directed to the outputmeans. In another aspect of the invention, the system provides sensingmeans for sensing light and is proximate the output means. The controlmeans is coupled to the sensing means and adjusts the optical meansbased on light sensed by the sensing means.

Many of the attendant features of this invention will be more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description and considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of an optical transmission apparatushaving an individually addressable multi-wavelength laser array directlycoupled to a 1:N micro-mechanical switch;

FIG. 2 illustrates one embodiment of an optical transmission apparatuswith a control system;

FIG. 3 illustrates another embodiment of an optical transmissionapparatus in which light is provided to a fiber via a movable mirrorbeing dynamically controlled;

FIG. 4 illustrates one embodiment of an optical transmission apparatuswith a magnetically movable mirror;

FIG. 5 illustrates one embodiment of an electrostatic movable mirror;

FIG. 6 illustrates one embodiment of an optical transmission apparatuswith a control system;

FIG. 7 illustrates a flow diagram of one embodiment of adjusting anoptical element to maximize fiber coupled power;

FIG. 8 illustrates a flow diagram of one embodiment of adjusting anoptical element to maximize fiber coupled power;

FIG. 9 illustrates a flow diagram of one embodiment of adjusting anoptical path to maximize fiber coupled power;

FIG. 10 illustrates a dithering control system and capable ofcontrolling various embodiments of an optical transmission apparatus;

FIG. 11 illustrates one embodiment of a control system using a chiseledfiber and capable of controlling various embodiments of an opticaltransmission apparatus;

FIG. 12 illustrates one embodiment of a control system using an anglecleaved fiber and capable of controlling various embodiments of anoptical transmission apparatus;

FIG. 13 illustrates one embodiment of a control system using a beamsplitter and capable of controlling various embodiments of an opticaltransmission apparatus;

FIG. 14 illustrates one embodiment of a control system using parasiticreflectivity and capable of controlling various embodiments of anoptical transmission apparatus; and

FIG. 15 illustrates one embodiment of a control system using a guidelaser and capable of controlling various embodiments of an opticaltransmission apparatus.

DETAILED DESCRIPTION

FIG. 1 shows an array of single frequency lasers, such as distributedfeedback (DFB) lasers, on a semiconductor substrate 5. The array oflasers comprises a number of independently addressable lasers 7. Eachlaser has a separate contact pad 3 from which current is injected intothe laser. Each laser is designed to operate at a different lasingwavelength, by, for example, varying the grating pitch in the laser oradjusting the effective index of the optical mode through varying thestripe width or the thickness of the layers that compose the laser. Whencurrent is injected into the laser using for example contact pads 3, thelaser emits radiation with a specific wavelength and from a particularposition on the chip, as represented by the arrows 9. In one embodiment,one laser is operated at a time, depending on the desired wavelength.The radiation or light from the lasers is transmitted to amicro-mechanical optical switch or switching element 11. The switchingelement has a number of states. In each particular state of a set ofstates, one of the input optical beams, i.e., light from one of thelasers, is transferred to the output 13 and transferred to the outputfiber l7. The entire assembly is packaged together on one submount 19.

The fabrication of multi-wavelength laser arrays is relatively wellknown in the art. To assign different wavelengths to each laser, anumber of techniques can be used, such as directly-written gratings withelectron beam lithography, stepping a window mask during multipleholographic exposures, UV exposure through an appropriately fabricatedphase mask, or changing the effective index of the mode of the lasers.Generally, for stable single mode characteristics, either a controlledphase shift is also included in the laser or gain/loss coupling is usedin the grating. The wavelength of such lasers can be accuratelycontrolled through dimensional variables, and varied across the array.

The lasers, switching element, and other components are more fullydescribed in the commonly assigned patent application Ser. No.10/000,142 entitled Tunable Controlled Laser Array, filed on Oct. 30,2001, now U.S. Pat. No. 6,914,916, the disclosure of which isincorporated by reference.

One exemplary embodiment of the switching element 11 is shown in thesystem of FIG. 2. In the embodiment illustrated in FIG. 2, current isprovided to a laser element of the laser array, e.g., laser element 7,and thereby the laser element emits light. The light from the laserelement is then directed by an optical element 25 to an optical output,e.g., an optical fiber 15. The optical element, in one embodiment, is amoveable mirror. The optical element, in various other embodiments, is acombination of mirrors, lens, beam splitters or other types ofstationary and/or movable optical components, such that light from anyparticular laser or one or more lasers is directed to the optical fiber.Additionally, the optical element, in various embodiments, directs lightor a portion of the light to a detector 23. In other embodiments, theoptical output provides light or a portion of the light to the detector.

Over time or due to certain conditions, e.g., thermal effects orpackaging disturbances, the laser to fiber coupling arrangement mayrequire alignment or adjustment.

A controller 21 with the detector 23 identifies and corrects formisalignments. The detector 23 senses, in various embodiments, aposition of the light beam or a measure of the power of light beam at apredefined position. In one embodiment, the detector 23 determines thepower of the light focused into the optical fiber. The controllerreceives the positional and/or power information from the detector, anddetermines if the fiber coupling arrangement should be adjusted. Assuch, the controller, in one embodiment, generates a signal used toadjust the fiber coupling arrangement. In one embodiment, the controlleris a digital signal processor configured to receive and interpret powerand/or positional information from the detector and to generate andissue adjustment commands to the optical element.

In FIG. 3, one embodiment of an optical element of FIG. 2 is shown. Thelight from laser element 7 is collimated by a lens 81 and strikes amoveable mirror 83. The mirror reflects the light to a lens 805 whichfocuses the light into an optical output, e.g., an optical fiber 15.

The fabrication of micro-mechanical tip/tilt mirrors, such as the mirror83, are well known in the art. Both surface micromachining techniquesand bulk silicon etching have been used to make such mirrors. Ingeneral, the precision required for a mirror used with the presentinvention is considerably less than that of large cross connectswitches, as the modes of the laser array are closely spaced. Thus, thepointing accuracy for the optical apparatus is considerably reduced.

In one embodiment, controller 21 consults a look-up table to determinean initial position of the mirror upon selection of a laser, andthereafter induces slight movement of the mirror to determine apreferred position. For instance, the controller maintains a lookuptable of the mirror positions in conjunction with the selection of eachof the lasers in the laser array. A detector sensing light from themirror or the optical fiber provides a signal to the controller, thesignal providing positional and/or power information regarding thelight. Based on the values in the lookup table and the measurementsperformed by the detector, the controller determines which direction themirror should be moved in order to provide optimal output power. Thus,as appropriate, the controller produces a control signal to move themirror, for example, in a first or second direction. The seconddirection is a direction that is substantially opposite from the firstdirection. The mirror, in one embodiment, is continually commanded towander and the output power monitored to compensate for movement ofcomponents of the package, thermal effects and other causes of potentialmisalignment and thereby provide maximum output power.

In a further embodiment, the controller determines if the positionaland/or power information from the detector differs from a predeterminedoptimal positioning of the arrangement and/or a predefined maximum oroptimal optical power of the light. Based on positional and/or powerdifferences determined by the controller, the controller moves themirror. For example, if the detector indicates that the light is atposition X1, and the controller, by referring to the look-table,determines that the light should be at position Y1, the controllercauses the mirror to move. As such, when the mirror is moved, the fibercoupling arrangement is adjusted. In other words, the mirror reflectsthe light from the laser and to the optical fiber, but to a differentpoint or position. In this manner, the detector and controller measurethe light to the optical fiber and adjust the mirror accordingly.Therefore, misalignments are corrected and optical power of the light atthe optical fiber is maximized. Various embodiments of the detectordetermining the positional and/or power information regarding the lightinto the optical fiber and the controller adjusting the mirror or otheroptical elements is described in greater detail later.

FIG. 4 shows a schematic of one embodiment of an optical system with amagnetically moveable mirror. In the diagram a laser array chip 5comprises a number of different laser elements 7, each of which has adifferent set of characteristics. Depending on the system requirements,the light from one particular laser element is used and directed by theoptical train to an output fiber 15.

In the particular embodiment illustrated in FIG. 4, the light from thelaser element is collimated by a fixed focusing lens 81 and impinges ona mirror 83. Once reflected from the mirror, the light is focused by asecond lens 805 and is coupled to the output fiber. In one embodiment,the collimating lens and the second lens are replaced by a single lens.The single lens is positioned approximately in the position of thecollimating lens of FIG. 4, with the mirror located at the back focalplane of the lens.

The mirror's rotation angle is adjusted both to select the beam of aparticular laser, and also to maintain the optimal coupling to theoptical output. Two magnets 85 attached to the rear of the mirror arepositioned within solenoids 87. Wires 89 are attached to the solenoids.A control current applied through the wires 89 controls the magneticfield which pulls one magnet into the solenoid and pushes the othermagnet out. Together with a fixed pivot point 803 and a spring 801, theangle of the mirror is tuned, i.e., tilted, using the control current.

FIG. 5 illustrates another embodiment of a moveable mirror. The mirrorhas three sections, a first section 151, a second section 153 and athird section 155. The first section 151 is stationary and is coupled tothe second section 153 via torsion hinges 151 a and 151 b. The secondsection rotates about torsion hinge 151 a, i.e., about a first axis. Thethird section is coupled to the second section via torsion hinges 153 aand 153 b. As such, third section also rotates about the first axis whenthe second section rotates. Additionally, the third section rotatesabout the torsion hinges 153 a and 153 b, i.e., about a second axis. Thethird section is also coated with or is made of a reflective material sothat light from a laser is reflected off the third section. By movingthe second and third sections about the respective first and secondaxes, the mirror is able to direct light from a laser to a multitude ofpositions.

In one embodiment, portions of the sections are each plated or otherwisemade conductive. From an external source (not shown), voltage is appliedbetween the plated portions and a substrate below the mirror. As such,the portions act as capacitor plates and thus an electric field isgenerated. Through the interaction of the charge on the mirror and theelectric field generated, a force is generated, such that the sectionsmove or rotate. The amount of force generated is based on the distanceor gap between the portions. In other embodiments, thermal actuators areused to position the mirror.

FIG. 6 illustrates another embodiment of a control system in accordancewith the present invention. An array of lasers 5 is formed on asubstrate. In the embodiment described twelve lasers are provided, andeach of the lasers produces light at a different wavelength, with thewavelengths centered around 1550 nanometers which is useful for fiberoptic telecommunications. The lasers are distributed feedback (DFB)lasers, although in different embodiments the lasers are distributedBragg reflector (DBR) lasers or vertical cavity surface emitting lasers(VCSELS). The VCSELs may be arranged linearly, as are the DFBs and DBRsin preferred embodiments, but the VCSELs are generally arranged in a twodimensional array.

Light from the lasers is passed through a collimating lens 61 and thento a moveable MEMS structure 63. As illustrated the MEMS structure is atwo axis tilt mirror, such as described in U.S. Provisional PatentApplication No. 60/309,669, entitled MEMS Mirror, filed Aug. 2, 2001,the disclosure of which is incorporated by reference. The mirror ismoved via a MEMS control 65 by applying voltages to contact pads,resulting in rotation of the mirror in what for convenience will bedescribed as the x and y axis. As illustrated, the light is thenreflected from a second mirror 67 to a fiber 15. In alternativeembodiments a prism is used to cause the light to reach the fiber. Usingeither the second mirror or the prism allows the laser and othercomponents to be packaged in a butterfly package of the type generallyused for laser light sources in telecommunications systems.

In various embodiments an optical isolator 69 is placed between thesecond mirror and the fiber. The optical isolator prevents, for example,stray reflections, from the end of the fiber or from discontinuities inthe telecommunication's line, from returning to the laser. Also invarious embodiments a modulator is placed after the optical isolator, orin its place, to modulate the light with an information signal.

In one embodiment a quad detector 601 is placed between the secondmirror and the fiber. The light from the second mirror is reflected ontothe quad detector which generates photocurrent in the four sections A,B, C and D. The ratio of these currents are stored and used formaintaining alignment. For instance, the ratio of the currents generatedin sections A and B of the quad detector are measured and stored. Also,the ratio of the currents generated in sections C and D of the quaddetector are measured and stored. An electronic control loop is thenconfigured to maintain these ratios during the operation of the device.When the device is first packaged, a calibration procedure is performedin which the currents to the different sections of the quad detector aremeasured when the beam is optimally aligned. These current values arestored and used in the operation of the device. By operating the MEMsmirror in a feedback loop to keep the ratios of these currents the same,the optical beam will point in the same direction and thus maximum fibercoupling will be maintained. In one embodiment, generated photocurrentsare provided to the MEMS control 65. Based on the generatedphotocurrents, the MEMS control produces an x axis control signal and ay axis control signal. Using these control signals or a signal orsignals representative thereof the MEMS control positions the mirror.

In other embodiments, the quad detector is placed behind the secondmirror or a third mirror is provided to direct light to the quaddetector. Various operation and placement of the quad detectors andphotodetectors relative to the other components in the package are alsodiscussed, for example, in U.S. Provisional Patent Application No.60/244,789, the disclosure of which is incorporated by reference.

In one embodiment, external to the package is an optional wavelengthlocker 603. As illustrated, the wavelength locker is an inlinewavelength locker, although in various embodiments the wavelength lockeris connected to the fiber by a tap. The wavelength locker illustrated inFIG. 6 determines the strength of light at two wavelengths about aselected wavelength. This is done, for example, by reflecting a portionof the transmitted light to two photodetectors. The transmitted light toa first of the photodetectors is of a wavelength slightly below theselected wavelength. The transmitted light to a second of thephotodetectors is of a wavelength slightly above the selectedwavelength. Various wavelength lockers are known to those of skill inthe art.

Generally, a wavelength error signal is formed using the ratio of theoutput of the two photodetectors. For example, the output of the firstphotodetector may be considered as forming the numerator, and the outputof the second photodetector may be considered as forming thedenominator. In such a circumstance, it may be seen that if thewavelength is too high the ratio will increase, and if the wavelength istoo low the ratio will decrease. Formation of the ratio, or a signalindicative of the ratio, may be accomplished using comparators,differential amplifiers, calculation by a microprocessor (followinganalog-to-digital conversion), or the like. The wavelength error signalis used for slight adjustments to the wavelength of the laser, using forexample temperature tuning, particularly for DFB lasers, or chargeinjection for DBR lasers.

In the embodiment described the output of the photodetectors in thewavelength locker is also used as an indication of output power from thelaser. The output of the photodetectors, for example, is summed by asummer. The output of the summer is an output power indicator, and isprovided to a control element. The control element produces a x axiscontrol signal and a y axis control signal using the control signal, ora signal or signals representative thereof. The x axis control signaland the y axis control signal is used to position the mirror.

In one embodiment the control element maintains a lookup table of mirrorpositions for selection of each of the twelve lasers. The lookup tableis populated, in one embodiment, at the time of manufacture of thepackage. On receipt of a command to select a particular laser, thecontrol element reads the appropriate values from the lookup table andgenerates the corresponding x axis control signal and y axis controlsignal.

Due to movement of components of the package, thermal effects, and othercauses of potential misalignment, the control signals generated usingthe lookup table may not appropriately position the mirror. Accordingly,in one embodiment the mirror position is commanded to wander slightly,with the output power indicator monitored to determine the mirrorposition providing maximum output power.

In another embodiment, the light from a tap on the fiber is provided toa photodetector. The photodetector produces a signal that isproportional to the output power of the light from the tap and isprovided to the controller. The controller adjusts the mirror based onprevious signals provided by the wavelength locker or an initialcalibration.

A flow diagram of a process for determining position of the mirror isprovided in FIG. 7. In Block 71 the process determines a selected laserbased on a laser select command. In Block 73 the process determines theappropriate control signals using a lookup table of expected mirrorpositions for selected lasers. In Block 75 the process samples the poweroutput indicator and stores the result as an initial result.

In Block 76 the process determines if the laser/fiber coupling positionshould be optimized. It may be desirable to interrupt the optimizationprocess occasionally (e.g., when switching between lasers).

In Block 77, the process alternates between, the first and second axes,e.g., the X and Y axes. Each axis position is optimized alternately toensure that an optimal position is maintained for both axes. In Block 78the optical element, e.g., the MEMS, is moved by a value of DELTA, whichis appropriately chosen for each axis according to the current positionof the MEMS. In one embodiment, the value of DELTA is determined from afunction of the MEMS position and an amount voltage used to move theMEMS, such that smaller values of DELTA are used for larger MEMSvoltages.

In Block 79 the process again samples the power output indicator andcompares the result with the initial or previous output power value. Ifthe comparison indicates a greater output power at the new position ofthe MEMS, the process replaces the x axis position in the lookup tablewith the new position in Block 81. If the comparison indicates lessoutput power at the new position, the process commands the mirror tomove to another position by a slight amount in a direction opposite theprevious direction. The process then samples the output power indicatorand compares the result with the initial result. If the comparisonindicates a greater output power at the new position, the processreplaces the x axis position in the lookup table with the new positionin Block 81.

Blocks 77 through 81 are repeated for the y axis, with the offsets beingin a second direction and a direction opposite the second direction. Theprocess then returns to Block 77, unless Blocks 77 through 80 indicate aposition of maximum power is attained. If the new position results in alower power, the direction of movement of the MEMS is reversed, e.g., bychanging the sign of DELTA, and repeating the loop. In Block 81, theinitial position is periodically updated in the lookup table using thecurrent position.

In one embodiment, however, the process repeats until a new laser, or nolaser, is selected. Continually repeating the process is beneficial, forexample, if thermal or other effects result in displacement of some orall of the system components. In addition, at initial laser selection,or whenever the wavelength of the laser is being adjusted, deviations inthe outputs of the photodetectors may be observed. Accordingly, in oneembodiment mirror positioning is not accomplished if the ratio of thephotodetector signals is outside a predefined limit. In anotherembodiment, mirror positioning as described above is first accomplished.Subsequently, the wavelength of the laser is adjusted and deviations inthe outputs of the photodetectors are observed.

In yet a further embodiment, the amount of movement of the mirror duringalignment is reduced as the process repeats. This allows, for example,for finer adjustment of the mirror position over time, and also helpsavoid limiting the mirror position to sub-optimal locations.

A flow diagram of another embodiment of the process for determiningposition of the mirror is provided in FIG. 8. In block 201 the processdetermines a selected laser based on a laser select command. In block203 the process using a lookup table determines the expected or initialmirror positions for the selected laser. In block 205 the processdetermines if the laser/fiber coupling position should be optimized. Itmay be desirable to interrupt the optimization process occasionally(i.e. when switching between lasers).

In block 207 the process samples the power output indicator and storesthe result as an initial result.

In block 209 the optical element, e.g., the MEMS, is moved in a selectedaxis by a value of DELTA and the power output is sampled again. In block211 the process calculates the slope of a power function by determiningthe change in the power output, i.e., the sampled power output in block209 minus the initial result obtained in block 207, over the change inthe position or location of the mirror in the first direction. In oneembodiment, the MEMS position is updated and the MEMS is moved in block214 by changing the current location by the calculated slope multipliedby a change factor.

In Block 216, the initial position is periodically updated in the lookuptable using the updated position of the MEMS determined in block 214. InBlock 218, the process switches to the other axis, i.e., alternatesbetween the X and Y axes. As such, each axis position is optimizedalternately to ensure the optimal position is maintained for both axes.

In one embodiment, however, the process repeats until a new laser, or nolaser, is selected. In yet a further embodiment, the amount of movementof the mirror during alignment is reduced as the process repeats. Thisallows, for example, for finer adjustment of the mirror position overtime, and also helps avoid limiting the mirror position to sub-optimallocations.

In another embodiment, predetermined positions of the mirror for the xand y axis and the power output of the laser selected are provided. In afurther embodiment, the process determines the power output relative topositions in both the x and y axis. In either embodiment, the processdetermines or approximates a power function that relates power output tothe positions of the mirror in the x and y axis.

For example, a tangent plane is determined in which the slope of thepower function relative to the position of the mirror in the x directionand the slope of the power function relative to the position of themirror in the y direction is determined for a specific position of themirror in the x and y axis, i.e., the point of tangency of the function.From the calculated function a local maximum is determined, i.e., wherethe power output is greatest and is neither increasing or decreasing ata specific position of the mirror in the x and y axis. In other words,the derivative of the power function is 0 in every direction. In oneembodiment, the local maximum determined is set as the initial startconditions or position of the mirror in the x and y axis in whichmaximum power output for the selected laser is obtained. The process inFIG. 7 is then used to confirm that this maximum power output point hasnot changed due to an error condition, such as movement of components ofthe package, thermal effects, etc.

A flow diagram of one embodiment of the process for determining positionof the mirror using a quad detector is provided in FIG. 9. In block 301the process determines a selected laser based on a laser select command.In block 303 the process samples the currents generated from eachsection A, B, C and D of the quad detector. In block 305 the processcalculates ratios of the currents from one section relative to anothersection. For example, in one embodiment, a first ratio (the current fromsection A over the current from section B) and a second ratio (thecurrent from section C over the current from section D) are calculated.Various other embodiments of the first and second ratios, such as thecurrent from section D over the current from section A, additionalratios, such as a third ratio (the current from section B over thesection C) and any combination thereof may be provided.

In block 307 the process analyzes the ratios in relation to the positionof the mirrors. In other words, if the process in block 307 determinesthat the first and second ratios are not equal to one, the process inblock 309 moves the mirror using a control signal. If the first ratio isgreater than one, the process moves the mirror along the x axis by apredetermined amount in a first direction. Alternatively, if the firstratio is less than one, the process moves the mirror along the x axis bya predetermined amount in a direction opposite of the first direction.If the first ratio is equal to one, but the second ratio is greater thanone, then the process moves the mirror along the x axis by apredetermined amount in a first direction. Alternatively, if the secondratio is less than one, the process moves the mirror along the x axis bya predetermined amount in a direction opposite of the first direction.The amount of movement is related to how much the measured ratio isdifferent from the ideal ratio. The process is then repeated continuingto block 303 sampling the currents from the quad detector. However, ifin block 307 the process determines that the first and second ratios areequal to one, the process ends. The process is also repeated startingfrom block 303 for the y axis with the movement of the mirror along they axis being in a second direction and a direction which is opposite tothe second direction. In one embodiment, the ratio corresponds to avalue determined during an initial calibration.

In one embodiment, however, the process repeats until a new laser, or nolaser, is selected. In yet a further embodiment, the amount of movementof the mirror during alignment is reduced as the process repeats. Thisallows, for example, for finer adjustment of the mirror position overtime, and also helps avoid limiting the mirror position to suboptimallocations.

In one embodiment, initially, the process using a lookup tabledetermines the expected or initial mirror positions for the selectedlaser. This lookup table is generated specifically for the device in aninitial calibration procedure after the laser is packaged. In anotherembodiment, a predetermined relationship between a section or sectionsof the quad detector and the position of the MEMS structure is provided.For example, more light on section A and thus more current from sectionA indicates that the mirror should be moved along the x axis in a firstdirection. Using the first example and the first ratio describe above,the process then in block 307 would recognize that the MEMS is to bemoved in the first direction in block 309 if the first ratio is greaterthan one. In another example, more light on section D and thus morecurrent from section D indicates that the mirror should be moved alongthe y axis in a second direction. Using this example and the secondratio describe above, the process then in block 307 would recognize thatthe MEMS is to be moved in the second direction in block 309 if thesecond ratio is less than one. As such, in this embodiment, if arelationship between the currents from the sections and the movement ofthe MEMS for the x and y axis is predetermined, the process does notneed to be repeated for the y axis.

A schematic for one embodiment of two axis control of an opticalapparatus 91 is shown in FIG. 10. The optical system has two controls tomaintain coupling, labeled X and Y on the diagram. The controlscorrespond to an electronic control of position, such as described inreference to the previous figure. For example, the x-control determinesthe rotation of a tilt mirror in the x-axis, while the y-controldetermines the rotation of the tilt mirror in the y-axis.

As illustrated in FIG. 10, an optical output of the unit is coupled to afiber 93. Some of the power is monitored through a tap 95 provides asignal to a photodetector 97. The higher the coupled optical power, thegreater the signal produced by the photodetector. The electrical signalproduced by the photodetector is provided to two phase lock loops orlock-in amplifiers 99 a and 99 b. These measure the sinusoidalcomponents present in an input signal at a frequency corresponding to areference input signal and generate a signal whose magnitude correspondsto the in-phase component of the input signal compared to the referenceinput signal. For example, if the sinusoidal component of the inputsignal is in phase with the reference signal, an in-phase output of thelock-in amplifier would be positive, while if the input signal isout-of-phase with the reference signal, the in-phase output would benegative.

The output of the lock-in amplifier is then provided to a respectivevariable signal source 901 a and/or 901 b, where it is integrated togenerate a DC signal. The DC signal is added to a sinusoidal AC source903 a and/or 903 b, and the combination is fed to the appropriatecontrol input of the optical system.

For two axis control, the AC sources for the x and y axes operate at twodifferent frequencies f₁ and f₂. If the positive cycle of the ditheringAC signal improves the optical coupling, then the DC signal willincrease to improve the output power, while if the positive cyclereduces the coupling, then the DC signal will decrease. The operation ofsuch a control loop falls under the domain of feedback analysis, and thetiming and stability can be easily calculated. Additionally, the DCsignals in various embodiments, are used to monitor the degradation ofthe package's fiber coupling and/or be used to warn of an impendingfailure. This is in contrast to conventional monitor photodiodes inlaser packages which monitor only the health of the laser chip itselfand not the fiber coupled power, which requires a costly external tap.

For telecom applications, such dithered signals may not be an importantissue. For example, in 2.5 Gb/s communications (OC48), the communicationlink generally has a low-pass cut-off of about 70 MHz. Thus lowfrequency oscillations on the output signal should not lead tosignificant errors in the data communication. Since the timing for theservo loop is ultimately limited by the mechanical time constants of thetransducer, the dithering frequencies and the response time of the loopis far slower then the low pass cut-off frequency.

An alternative method of dithering that does not depend on phase lockedloops or two different dithering frequencies, as described above, is todirectly tilt the beam in other directions and monitor improvements orchanges in fiber coupled power. For example, if the lookup tableindicates that previously the optical position for the mirror was at anx voltage of 100V and a y voltage of 50 volts, the microprocessordetermines and utilizes five points (100,50), (100.1, 50), (99.9, 50),(100,50.1), and (100, 49.9), and then takes the point with the highestpower for the next iteration. In one embodiment, the alternative methodis performed by a fast microprocessor in a telecommunication systemhaving a good signal to noise ratio.

However, in many applications, having a dithered signal on the output isnot acceptable, or splicing an appropriate tap on the output fiber maybe prohibitive. In such cases an alternate method of generating an errorsignal for the control loop is used. One method is described below inreference to one embodiment of a fiber arrangement shown in FIG. 11. Inplace of a standard cleaved fiber, a chiseled fiber is used. Such fibersare readily built by a number of suppliers. The fiber 105 has a flatsection where a single mode region is contained, but has slopingsidewalls. When the light is focused by lens 101 on the core, some ofthe light at the edges of the beam is incident on the sloping sidewallsand is reflected around the fiber. There are four photodetectors placedon the periphery of the fiber, photodetector 103A is above the fiber,103B below, 103C to the right and 103D to the left.

In the initial packaging stage, the electronic control is varied tooptimize the coupling to the fiber. When this optimal packagingcondition is achieved, the ratio of the optical power falling on thevertical detectors (power incident on photodectector 103A divided by thepower falling on photodetector 103B) and on the horizontal detectors(power on the photodetector 103C divided by the power falling on thephotodetector 103D) is measured and the value stored. An electroniccontrol loop, such as that described in FIG. 9, is then configured tomaintain these ratios during the operation of the device.

For example, if a different laser is selected, the electronic controlloop adjusts to once again achieve these ratios. By using twophotodetectors for each axis, an error signal independent of the opticalpower can be obtained. In another embodiment, three photodetectors areused along with a more complex control circuitry. For example, in FIG.11, photodetector 103A can be left out. Lateral control is achieved byadjusting the ratio between the detected signal from photodetectors 103Cand 103D, while vertical control is achieved by adjusting the ratiobetween a detected signal at photodetector 103B and the sum of detectedsignals from photodetectors 103C and 103D. The embodiment of FIG. 11 canalso be combined with the dither approach mentioned previously, with invarying embodiments differing numbers of detectors being used. In oneembodiment, a single detector provides an adequate feedback signal, withmaximum coupling to the fiber occurring when the least amount of lighthits the sloped sides and scatters out. As such, with a single detector,by minimizing the amount of scattered light, maximum coupling to thefiber can be achieved.

An alternative approach to using a chiseled lens is shown in FIG. 12.For most fiber coupling applications where minimal feedback is requiredfor both the active component (such as a DFB laser) or for the system,an angle cleaved fiber is used. When the light is focused on the fiber,there is a parasitic reflection from the cleave. The reflection isrefocused on a position-sensitive or a quad detector that will producean error signal. Like the previous embodiment shown in FIG. 11, therelative ratios of light on the detectors, or the signal correspondingto the position can be stored as an initial calibration. Any offset fromthe initial calibration is detected by the electronics and fed back intothe control loop. In FIG. 12, light is focused by lens 101 onto thefacet of an angle cleaved fiber 113. The image on the cleave is thenrefocused by a second lens 115 onto a quad detector 117. This generatesphotocurrent in the four sections, labeled A, B, C, and D. The ratio ofthese currents are stored and used for maintaining alignment, aspreviously described with reference to the photodetectors in FIG. 11.

If the quad or position-sensitive detector is placed very close to thefiber, then the second refocusing lens 115 can be eliminated, as anyshift in the position of the image on the fiber will translate tovarying detected powers on the photodetector. The same concept can beapplied to coupling to other structures. For example, when coupling alaser to a lithium niobate chip, the position of the reflection from thefacet can also be monitored.

In one embodiment, a beam splitter is placed before the optical fiber toindependently generate a second image. This is shown schematically inFIG. 13. The focusing lens 101 images light onto the end face of a fiber123. Between the lens and the fiber is a beamsplitter 121, which forms asecondary image on a quad or position-sensitive detector 125. Onceagain, the ratio of generated photocurrent in the detectors is storedduring an initial calibration process, and an electronic feedback systemmaintains this ratio continuously during the operation of the device.

In general, any optical system will have a set of parasitic reflections,and these can also be used to maintain optimal coupling to the fiber.FIG. 14 shows that a reflection from the back side of the movable mirrorforms a secondary image next to the fiber if the mirror is not perfectlyparallel. One could also consider the front side reflection asparasitic, and the backside reflection as the main beam, depending onhow the mirror is coated. The light beam from the laser element 7 vialens 81 reflects both from a front face of the mirror 133 and also froma back face 135. The image from the front face is focused on a fiber 15via lens 805, while the image form the back face will occur slightlydisplaced from the main image. A quad or position-sensitive detector 131is placed at this secondary image to lock the main image on the fiber,as previously described. In various other embodiments, other parasiticimages are used, such as a reflection from other optical elements, forexample, the exit window at one end of the package, or the lighttransmitted through the mirror which shifts as a function of mirrorposition.

For optical systems with laser arrays, as described in the figures,whenever a particular laser needs to be coupled to the fiber, the laseron one side of the active device is also activated, though perhaps at alower output power. This adjacent laser source produces an adjacentimage next to the fiber core. The relative position depends on thespacing in the array and the magnification of the optical system. Sincethe beam from this laser is not needed in the fiber, a quad detector ora position-sensitive detector is used to detect the position of theadjacent image. As previously described, an error signal is generated bymeasuring the ratio of photocurrents in the quad detector. However, inusing this technique, an additional laser is utilized and extra power isconsumed for this additional “guide” laser. This embodiment is describedin further detail in reference to FIG. 15.

The multi-element laser array contains lasers of differentcharacteristics. A particular laser, e.g., laser element 7, is selectedto be coupled into the fiber 15. An adjacent laser, e.g., laser element143, is also activated, and the beam from the adjacent laser propagatesthrough the optical system, i.e., via lens 81, mirror 145, and lens 805,and is focused adjacent to the fiber. A position sensitive detector 141detects the location of the adjacent image or guiding spot to generatethe error or feedback signal. In other embodiments, instead of theadjacent laser being used, others lasers, a given distance away from theselected laser, are used. Using a larger separation between the “active”laser and the “guide” laser can be helpful, as the image is further fromthe fiber core and thus easier to capture from a photodetector. However,a number of lasers on the chip may be required. For example, if twelvelasers are used and a third laser away from a selected laser along onedirection of the laser array is used to monitor the fiber coupling, thenfifteen lasers in total would be used on the chip.

Accordingly, the present invention provides a system and methodology forcontrolling fiber coupling between tunable lasers and an optical outputto maximize fiber coupled power. Although this invention has beendescribed in certain specific embodiments, many additional modificationsand variations would be apparent to one skilled in the art. It istherefore to be understood that this invention may be practicedotherwise than is specifically described. Thus, the present embodimentsof the invention should be considered in all respects as illustrativeand not restrictive. The scope of the invention to be indicated by theappended claims, their equivalents, and claims supported by thisspecification rather than the foregoing description.

1. An optical apparatus comprising: an array of lasers on asemiconductor substrate, each of the lasers in the array of lasersdesigned to emit light at a wavelength different than other lasers inthe array of lasers; a microelectromechanical (MEMS) mirror moveable toreceive light generated from any one of the lasers of the array oflasers and to direct the received light from the one of the lasers ofthe array of lasers towards an output path; an optical fiber configuredto receive at least a portion of the light from the one of the lasers ofthe array of lasers in the output path; a beamsplitter in the outputpath between the MEMS mirror and the optical fiber, the beamsplitterconfigured to direct some of the light from the one of the lasers of thearray of lasers out of the output path; a position-sensitive detectorconfigured to receive at least some of the light generated from the oneof the lasers of the array of lasers directed out of the output path;and a controller configured to receive a signal representative of aposition of the light received by the detector and to generate a controlsignal for use in positioning of the MEMS mirror.
 2. The opticalapparatus of claim 1 wherein the detector is a quad detector.
 3. Theoptical apparatus of claim 2 wherein the controller is configured togenerate the control signal to position the MEMS mirror based on ratiosof power received by different sections of the quad detector.
 4. Anoptical transmission system comprising: a plurality of lasers in amulti-wavelength laser array on a common semiconductor substrate, thelasers being individually addressable with each of the plurality oflasers designed to emit light at a wavelength different than the otherlasers; an optical element comprising: a collimating lens configured tocollimate light from the multi-wavelength laser array; amicro-mechanical mirror positionable to direct light from a selectableone of the lasers of the multi-wavelength laser array; and abeamsplitter to split light from the selectable one of the lasers of themulti-wavelength laser array directed by the micro-mechanical mirrorinto at least two portions; an optical fiber to receive a first portionof the light from the selectable one of the lasers of themulti-wavelength laser array split by the beamsplitter; aposition-sensitive detector to receive a second portion of the lightfrom the selectable one of the lasers of the multi-wavelength laserarray split by the beamsplitter; and a controller configured to receivea signal indicative of a position of light received by the detector andto provide a signal indicative of desired micro-mechanical mirrorposition.
 5. The optical transmission system of claim 4 wherein thedetector is a quad detector.
 6. The optical transmission system of claim5 wherein the controller is configured to provide the signal indicativeof desired micro-mechanical mirror position to position themicro-mechanical mirror based on ratios of power received by differentsections of the quad detector.